See the current Perl documentation for bless.
Here is our local, out-dated (pre-5.6) version:
bless - create an object
This function tells the thingy referenced by REF that it is now an object in the CLASSNAME package--or the current package if no CLASSNAME is specified, which is often the case. It returns the reference for convenience, because a bless() is often the last thing in a constructor. Always use the two-argument version if the function doing the blessing might be inherited by a derived class. See the perltoot manpage and the perlobj manpage for more about the blessing (and blessings) of objects.